Arguing a fortiori

Courts and lawyers often argue a fortiori. Sometimes they actually use the Latin phrase to indicate that their conclusions do not just follow, but “follow a fortiori” from certain premises. These inferences are taken to constitute a distinct and important kind. But how exactly are they distinct, and why are they important? That is less clear. Despite their popularity, a fortiori arguments are not particularly well understood, and so far they haven’t drawn much attention from legal theorists.

I try in this paper to make some progress on the topic. I will be pursuing two goals. The first is to bring out the form of a fortiori arguments, articulating those assumptions that, though typically left unstated, are necessary steps of arguments of this kind. The second goal is to say something about the point of such arguments, and to characterise the sort of context in which an arguer will have reason to deploy an a fortiori rather than an inference of a different sort.